Why do Internet Exchange Points Play a Fundamental Role in the Internet Ecosystem?
'21.1 A Short Answer' When you use the Internet, what happens? Whether you go online to chat with you friends, or watch a video on YouTube, or send an email, or buy a book on Amazon, or check the weather on AccuWeather, or search a topic on Wikipedia, it feels like there is one wire connecting you directly to the thing you want. But a billion other people may be connected to a billion other things at the same time. How can that happen? This is possible making agreements. The Internet is based on the principle of end-to-end reachability, meaning that any user can reach any other user as though they are in the same network. As a network of networks, the Internet is based on interconnections of independent networks, also known as'' Autonomous Systems'' (AS), which are owned by Internet Service Providers ''(ISPs), or telecommunications carriers, or content providers, or webhosters, or a combination of previous categories. Internet interconnections designate the global reachability and involve routing decisions based on path, network policies using an exterior gateway protocol, i.e. ''Border Gateway Protocol (BGP). There are two main approaches to interconnect Autonomous Systems (ASs) with each other for exchanging traffic and for global Internet reachability: transit and peering relationships. Transit arrangement, also known as a tiered interconnection, consists of a business relationship whereby one larger ISP (known as Tier 1 ISP or transit free), like Sprint, Level 3, or Verizon, sell access to their routing tables, in order that traffic sent from other ISPs (Tier 2 ISP) is delivered to the Internet. These Tier 2 providers, that has to purchase transit from Tier 1 ISP, can sell in turn access to other ISPs. The seller ISP, known as upstream ISP, also advertises its relationship with the buying ISP to guarantee its downstream providers gets traffic. The transit provider charges the downstream ISP on a metered basis, usually measured on a Megabit-per-second basis. Peering relationship usually is a free agreement, whereby ASs having similar traffic volumes interconnect each other and mutually exchange their resources. In this scenario there are Internet eXchange Points (IXPs) that offer a shared infrastructure, in order to allow ASes to interconnect with one another directly, via the exchange, rather than through one or more third-party networks. The direct interconnection leads to decrease network costs, to improve network performance and to make AS networks more redundant. Performance improvement is clearly obtained keeping local the local traffic, considering that many networks access directly at the exchange point, that otherwise would have taken several “hops” through other parties. Moreover, since in the IXP there are many routes through which traffic can be sent, many alternatives are available. Thus, an ISP can increase the redundancy in its network, not just relying on one point of interconnect or its transit provider. This reduces further the need for IXP members to send traffic through transits of upstream providers. In few words, IXP is a more scalable solution to transit problems. Although IXPs play a fundamental role in the growth of the Internet, several of their features and their impact on the evolution of the Internet have still not known for most of us. '21.2 A Long Answer' What we call “''the Internet''” is actually an interconnection of millions of network devices (routers, servers, workstations, etc.) operated by a set of network operators, content providers, and end users. These “players” are independent members of a system referred to as the Global Internet Peering Ecosystem. This is a community of loosely affiliated network service providers, that interact and interconnect their networks in various business relationships. A Peering Ecosystem is generally composed of three general classes of Internet players: Tier 1 ISPs, Tier 2 ISPs, and Content Providers. Each Player within these classes hold similar power positions within the Peering Ecosystem and therefore tend to have similar motivations and exhibit similar behaviors within the Peering Ecosystem. • Tier 1 ISPs do not do not have to pay transit fees since by definition they can reach all the networks in the Internet Peering Ecosystem solely through free peering relationships. They are at the top of the hierarchy and all other ISPs operating in the region are required to purchase transit from one (or more) of them, in order to reach all destinations in the Internet Region. • Tier 2 ISP is an ISP that purchases (and then resells) transit within an Internet Region. They often adopt peering to reduce transit costs, improve performance, and potentially even increase revenue. • Content Providers are all corporations that provide an Internet Service and are focused on content development. By definition, they do not sell access to the Internet. The simply purchase transit from an upstream provider and create content as their core business. Content Players include companies like Netflix, YouTube, Amazon, Akamai, eBay, and General Electric. '21.2.1 Historical Background' During the last few years, some significant events contributed to a deep change on the Internet Ecosystem. About in 2000, due to the Economic Collapse of the Telecommunication Sector, several most important Tier 1 ISPs and Upstream Provider went bankrupt. This forced several minor companies to negotiate expensive emergency Gigabit/transit connections. Another important aspect is related to the Peer-to-Peer Sharing Networks, which grow exponentially in popularity and the traffic between sharing users shifted from few MBs of music files to few GBs of movies. Since the sharing users belonged to different AS, the ISPs migrated soon into Exchange Points in order to peer with each other and to obtain a routing optimization. In this way, ISP offloaded this traffic from their transit connections (which they paid for) onto free peering interconnections. So the increase of P2P traffic induced some ISPs to converge in the IXP and make traffic strongly localized. In turn, peering induced applications like Torrent to prefer to fetch files across the recently peered network path. Torrent selection protocol uses latency to determine which Torrent file is “''more local''”. The result was a 20% growth in peering traffic volume, Torrent-based. Another interesting effect is related to the traffic exchanged with AOL, which represented the “''800 pounds Gorilla''”. In each IXP where AOL peers, the other ISPs were inclined to establish a PoP (Point of Presence). And this gravitational pull of the ISP, further accelerated the number of peers pulling into IXP, which in turn accelerated the volume of traffic, migrating from transit interconnections to peering ones. '21.2.2 IXPs and their participants' Internet eXchange Points offer a shared infrastructure for ASs to interconnect on an individual basis with all the other networks. Thus, the IXPs may be seen as physical hubs which allow ASs to exchange traffic with each other, as if they were connected directly. In what follows, we call participants the ASs which are connected to at least one IXP. ASs can be classified according to their geographical attributes as following: - national AS, if every its geographical location is placed in only one country - continental AS, if every its geographical location belongs to one continent (e.g., North America, Europe, Asia). - worldwide AS, if there are at least two geographical locations, that are placed in two different continents (e.g., Europe and South America). In latest years IXPs are considered as crucial components of the Internet’s AS level substrate, since they connect various networks with each other and exchange data in a reliable and efficient manner. Thanks to the IXPs, Internet traffic can be maintained localized in the geographical region it belongs. In fact, most of Internet traffic is directed inside the borders of the country where it is generated, and IXPs typically host a lot of regional ASs. This prevents that traffic between regional ASs passing through expensive connections (e.g. submarine fiber connections between Oceanic countries or satellite connections in Africa). This clearly leads to an improvement in network performance. In addition to technical benefits, for medium-sized ASs there are also economic advantages of belonging to an IXP. In fact, they can avoid multiple ad-hoc point-to-point connection costs among participants, which are otherwise needed when BGP works between them. '21.2.3 Measuring node importance' Now, we want to focus on IXPs that are fundamental for well-connected zones, since they lead a high number of peering relationships. We attempted to outline different AS behaviors using the classical graph theory. According to 4, we can obtain a model that could approximate the Internet, in order to predict several properties on AS-level topology, such as high tolerance to node failures, but also issues such as less robustness against attacks. Every node in the network is described by typical graph theory indices. We denote: • degree, which specifies the number of connections a node has (degree is often proportional to node importance); • clustering coefficient, which expresses the level of connectivity among a node neighbors; • betweenness centrality, which gives an idea of the node centrality. In the following section we will use the betweenness index to grade ASs, whose economic market is most probably not Internet-driven, i.e. they do not transit traffic for other ASs. To identify them, we search the graph for nodes with a betweenness value equal to 0. This value indicates that, if the Internet routing was shortest path driven, none of the ASs would use the considered node as a transit. We know that the BGP routing procedure is mostly economically driven and may be strongly different from a shortest path routing. Thus the betweenness index will not allow us to draw conclusions about traffic routing. Nevertheless, these kinds of nodes still play an irrelevant role on the graph, since their removal would not split the full Internet graph in any way. 'Example' In this example we want to show how different AS importance metrics turn out to be for the nodes in the hypothetical small network of an IXP, represented by the following graph: The first step to compute the required metrics is to write down the adjacency matrix of the given graph. We have: : A = \begin{bmatrix} 0 & 1 & 1 & 1 & 0 \\ 1&0&0&0&1 \\ 1&0&0&1&1 \\ 1&0&1&0&1 \\ 0&1&1&1&0\end{bmatrix}. From this matrix we can quickly find the degree of the nodes just calculating the matrix: A^2=A A . We have: : A^2=A A= \begin{bmatrix} 3&0&1&1&3\\ 0&2&2&2&0\\ 1&2&3&2&1\\ 1&2&2&3&1\\ 3&0&1&1&3\end{bmatrix}. Thus we have that the degree vector is diag(A^2 ) = \begin{bmatrix} 3&2&3&3&3&3\end{bmatrix}. . We can observe that node 2, gluing less nodes the graph together, is less important than the others. In order to compute the betweenness centrality of the nodes, we need to evaluate over how many shortest path the nodes are positioned. We denote as g_{st} the total number of shortest paths between two different nodes s and t , and n_{st}^i is the number of such paths that the node i sits on. Thus, we have: : G = \begin{bmatrix} -&0&1&1&3\\ 0&-&2&2&0\\ 1&2&-&2&1\\ 1&2&2&-&1\\ 3&0&1&1&-\end{bmatrix}. Let us now calculate the N_i matrix for each node i , which contains the number of shortest over which node i sit. For example, we have: : N_3 = \begin{bmatrix} -&0&-&0&1\\ 0&-&-&0&0\\ -&-&-&-&-\\ 0&0&-&-&-\\ 1&0&-&0&-\end{bmatrix}. for the node 3. Then the betweenness centrality of node is defined as: \sum_{m}\sum_{t ''References '1' B. Augustin, B. Krishnamurthy, and W. Willinger. “IXPs: mapped?”, IMC ‘09: Proceedings of the 9th ACM SIGCOMM conference on Internet measurement conference, pages 336–349, New York, NY, USA, 2009. '2' E. Gregori, A. Improta, L. Lenzini, and C. Orsini. “The impact of IXPs on the AS-level topology structure of the Internet”. Computer Communications, 2010. '3' CISCO. (2009) “The value of peering”. ISP/IXP Workshop. http://www.pacnog.org/pacnog6/IXP/IXP-peering.pdf '4' M. Faloutsos, P. Faloutsos and C. Faloutsos, “On power-law relationships of the Internet topology”, in Proc. ACM SIGCOMM, 1999, pp. 251-262 '5' M. Z. Ahmad, R. Guha, “Understanding the impact of internet exchange points on internet topology and routing performance”, Proceedings of the ACM CoNEXT Student Workshop, Philadelphia, PA, USA, 2010. '6' M. Chiang, “Networked Life: 20 Questions and Answers”. Cambridge University Press, 2012. '7''' W. B. Norton, The Evolution of the U.S. Internet Peering Ecosystem, http://drpeering.net/white-papers/Peering-Policies/A-Study-of-28-Peering-Policies.html